Electrochemical fuel cells convert chemical energy of fuels directly into electrical energy to provide a clean and highly efficient source of electrical energy. Like a battery, a fuel cell consists of two electrodes (an anode and a cathode) separated by an electrolyte typically made of a thin polymeric membrane. In a typical fuel cell, hydrogen gas from the fuel reacts electrochemically at the anode electrode and is converted into protons and electrons. The protons move through the electrolyte to the other electrode (i.e., the cathode), where they combine with the product from the reduction of oxygen from the air to form water, which is expelled from the cell as vapor. The involvement of hydrogen and oxygen in the two reactions, one releasing electrons and the other consuming them, yields electrical energy that is tapped from across the electrodes.
Because of the high conversion efficiencies and low pollution, fuel cells such as hydrogen and direct methanol fuel cells are becoming increasingly attractive power sources for mobile and stationary applications. Such applications include on-board electric power for advanced propulsion systems for non-polluting vehicles. While researchers around the world are developing potential fuel cell applications including electric vehicles and portable electrical power supplies, these developments face challenging scientific problems in the areas of materials science, interfacial science and catalysis. In proton exchange membrane fuel cells (PEMFCs), hydrogen ions must be transported through a semi permeable membrane. Hydrocarbon fuels must first be converted to pure hydrogen by reforming, and the overall conversion requires complex process technology. In addition, substantial investments must be made in safety and controls.
Direct methanol fuel cells (DMFCs) offer a simpler solution and require no reformer. Direct methanol fuel cells are increasingly considered an attractive power source for mobile applications because of the high energy density, the fuel portability, and the easily renewable feature of methanol. The fuel portability of methanol is particularly important in comparison with the difficulties of storing and transporting hydrogen.
In fuel cell reactions, both anode and cathode catalysts are very important for many reasons.
Anode Catalysts:
The readily-obtainable energy density (approximately 2000 Wh/kg) and operating cell voltage (0.4 V) for methanol fuel cells is presently lower than the theoretical energy density (approximately 6000 Wh/kg) and the thermodynamic potential (approximately 1.2 V) for such fuel cells. These problems are largely caused by poor activity of the anode catalysts and “methanol cross-over” to the cathode electrode. These problems account for a loss of about one-third of the available energy at the cathode and another one-third at the anode.
Pt-group metals have been extensively studied for both anode and cathode catalysts, but a major problem is the ease with which they may be poisoned by CO and CO-like intermediate species typically present. Binary PtRu nanoparticle catalysts on carbon supports are currently considered among the most promising catalysts. Binary PtRu catalysts exhibit a bifunctional catalytic mechanism in which Pt provides the main site for the dehydrogenation of methanol and Ru provides the site for hydroxide (OH) and for oxidizing CO-like species to CO2.
Recently, gold at nanoscale sizes was found to exhibit unprecedented catalytic activities, both for CO oxidation and for electrocatalytic activity for CO and methanol oxidation. Studies show that nanoscale gold-based bimetallic materials may provide a synergistic catalytic effect for the methanol oxidation reaction (MOR) at the anode in methanol oxidation fuel cells. For example, the synergistic catalytic effect of gold-platinum (AuPt) nanoparticles might suppress adsorbed poisonous species by changing the electronic band structure to modify the strength of the surface adsorption.
While bimetallic AuPt is a known electrocatalyst for oxygen reduction in alkaline fuel cells, few reports concern utilizing AuPt nanoparticles with controllable size and composition in fuel cell catalyst applications. In such bimetallic systems, Pt could provide the main hydrogenation or dehydrogenation sites, while Au together with Pt could speed up the removal of poisonous species. In the past, decreasing activation energy to facilitate oxidative desorption and suppressing adsorption of CO were believed to lead to sufficiently-high adsorptivity to support catalytic oxidation in alkaline electrolytes. However, it has recently been shown that catalysts prepared by impregnation from Pt and Au precursors provided results similar to those of monometallic Pt catalysts, suggesting that the presence of Au did not significantly affect the catalytic performance of Pt. This is attributed to phase-segregation of the two metals due to their miscibility gap. As such, only Pt participates in the adsorption of CO and the catalytic reaction. How the bimetallic catalytic properties depend on nanoparticle preparation and composition is an important area for the development of new or improved catalysts for fuel cell research.
Cathode Catalysts:
As previously stated, both the energy density and operating cell voltage for direct methanol fuel cells are currently much lower than values that are theoretically possible. At the cathode, the kinetic limitation of the oxygen reduction reaction (ORR) is a problem of interest in proton exchange membrane fuel cells operating at low temperature (<100° C.) and in DMFCs. The rate of breaking O═O bonds to form water strongly depends on the degree of the oxygen interaction with the adsorption sites of the catalyst and competition with other species in the electrolyte (e.g., CH3OH). There is also strong adsorption of OH forming Pt—OH, which causes inhibition of the O2 reduction.
The present inventors have recently investigated gold and gold alloy nanoparticles as potential electrocatalysts fuel cell reactions such as CO and methanol oxidation reactions and oxygen reduction reactions. The exploration of gold nanoparticles in catalysis shows potential applications in fuel cell related catalytic reactions. The nanoparticle surface properties are essential for the adsorption of oxygen and the catalytic reaction of gold at nanoscale sizes. Bimetallic AuPt composition may produce a synergistic catalytic effect that involves the suppression of adsorbed poisonous species and the change in electronic band structure to modify the strength of the surface adsorption for ORR.
The study of AuPt binary and AuPtM ternary nanoparticles with controllable size and composition for fuel cell catalyst applications is important because metal nanoparticles in the size range of approximately 1-10 nm undergo a transition from atomic to metallic properties, and the bimetallic composition could produce synergistic effect. For example, for the adsorption of (OH−)ads (i.e., OH− species adsorbed on the catalyst surface) in an alkaline medium the presence of Au in Pt catalysts could reduce the strength of the Pt—OH formation. A full understanding of how the synergistic catalytic effect operates at the nanoscale gold and gold-platinum surface remains elusive. Gold-based binary (AuPt) and ternary (AuPtM) nanoparticles of 1-10 nm core sizes with controllable Au, Pt and a third metal (M), for example, M=W, Ti, Cr, Fe, Mn, etc. have been prepared. These nanoparticles may be assembled onto high surface area carbon nanomaterials with controlled dispersion and loading. The carbon-supported AuPt/C or AuPtM/C nanoparticles are processed by thermal treatment to achieve desired characteristics including size, composition and alloy properties. The electrocatalytic activity of such thermally treated, carbon-supported nanoparticles may be evaluated in both methanol oxidation reactions (MOR) and oxygen reduction reactions (ORR).
The introduction of the third metal component into the binary nanoparticles forms AuPtM ternary nanoparticles. Ternary nanoparticles are expected to lead to several important modifications of the catalytic properties, possibly including further modification of the electronic structure, introduction of an oxide component on the catalyst surface via the propensity of oxide formation of the metal component, and control of the size increase during thermal treatment via alloying with the metal. The relatively low cost of the third component metal should help lower the cost of catalyst materials. Metals such as W, Ti, Pt, Cr, Fe, and Mn, all having melting points higher than Au may be used to prepare AuPtM nanoparticle catalysts. The research is coupled with combinatorial knowledge base for better design of binary and ternary nanoparticles with controllable size, composition and phase properties. The binary/ternary catalysts can be used not only as methanol-tolerant cathode catalysts in fuel cell membrane electrode assemblies (MEAs), but also as CO-tolerant catalysts in combination with the desired oxide support materials for water-gas shift reactions.